Referring to FIG. 1, a current single conversion tuner module 10 is shown. Single conversion refers to the number of frequency translations that the incoming signal is subjected. For example, in the U.S. the frequency plan for most cable networks span the frequencies from 54 MHz to 857 MHz. Each channel at the input of the tuner 10 spans 6 MHz for a total of 133 input channels. The tuner 10 selects one out of the multitude of channels and translates the selected channel to a fixed IF frequency of 44 MHz. Frequency translation is also commonly termed as conversion, hence the tuner 10 is often referred to as a single conversion system. The single conversion is achieved by mixing the incoming signals with a local oscillator signal (OSC) present inside the tuner 10. For example, if an incoming channel centered at 100 MHz is mixed with a 144 MHz local oscillator signal, the resultant signal at the output of the mixer 14 is a sum frequency product at 244 MHz and a difference frequency product at 44 MHz. The sum frequency product is typically eliminated by use of a SAW filter 16, centered at the desired output or Intermediate Frequency (IF), in this case at 44 MHz. SAW filters provide a high degree of selectivity to the incoming signals providing a significant level of attenuation to signals outside of the pass band. In the U.S., the SAW filter pass band is selected to be approximately one channel bandwidth (i.e., 6 MHz).
If in the above illustration, the channel at 100 MHz is defined as the desired channel, there also exists an image or undesired channel which could also mix with the local oscillator signal OSC and produce an output at the IF of 44 MHz. Consider an example of a channel at 188 MHz. If the channel were to be mixed with a local oscillator signal at 144 MHz, the channel could also produce a difference output at the IF of 44 MHz and a sum frequency output product at 188 MHz. The SAW filter 16 would attenuate the output at 188 MHz. However, the SAW filter 16 would not be able to distinguish between the output of the image channel mixing and the output of the desired channel mixing, both of which would be at the desired IF of 44 MHz. The single conversion tuner 10 overcomes an undesired channel by the use of the tracking channel filter 12 at the input of the tuner 10. The incoming signals pass through the tracking filter 12 before the mixer 14. Tracking filters are typically 20–40 MHz wide and eliminate the undesired image channel from being subjected to the mixing process, thereby ensuring that the output at the IF is only due to that of the desired channel.
For a given mixing step, there exists an undesired image channel spaced at twice the IF from the desired channel. While the use of input tracking filters greatly alleviates the image channel problem, input filters need to track the local oscillator frequency in order to ensure that the image rejection is maintained across the input signal band. Moreover, in cable modem systems, for proper operation each modem also needs to present a controlled input impedance across the input frequency band. The input tracking filters present a non-uniform input impedance across the input frequency range while attenuating the image channel. Typically, input tacking filters have tuned passive devices, which need to be manually tuned during the tuner module assembly process. Manual tuning is a significant portion of the manufacturing costs. To overcome the single conversion tuner drawbacks, tuner manufacturers have introduced tuner modules, which feature a dual conversion architecture.
Referring to FIG. 2, a typical dual conversion tuner module 20 is shown. In dual conversion tuners the frequency translation from the input frequency band of 48 MHz–857 MHz to the output IF of 44 MHz is achieved in two mixing steps. Nominally the first mixing step 24 involves upconverting the entire input frequency band to a first IF frequency (IF1) which is 1100 MHz. There are two desirable properties associated with this upconversion mixing. The first IF at 1100 MHz is out of band to the input channel frequency band. Also, the image channel for the first IF needs to be filtered out with a fixed low pass filter 22 at the input of the tuner 20. The low pass filter 22 would not have to be a tracking filter and could help present a controlled impedance to a cable network. If for example, the desired channel is located at 100 MHz, the first local oscillator signal frequency (OSCL) would have to be 1200 MHz for a subtractive upcoversion mixing step for a first IF of 1100 MHz. Since the tuner module 20 still has to have an output at 44 MHz, the second mixing step 28 downcoverts the signal at the first IF by mixing it with a second local oscillator signal (OSC2) at a frequency of 1056 MHz.
As in the single conversion tuner 10, there exists a SAW filter 30, which provides the desired channel selectivity at 44 MHz. However, the dual conversion tuner architecture has the following drawback. The image channel for the second mixing step 28 could still be present at the first IF output IF1. For example, in the above illustration if there is a signal present at the first IF IF1 at 1012 MHz, the signal too would downconvert and appear at the second IF output IF2 at 44 MHz. The SAW filter 30 would not be able to distinguish such a signal from the desired channel. Therefore, the filter 26 at the first IF of 1100 MHz should have a narrow pass band or a high enough Q (quality factor) to suppress the signal at the image frequency of the second mixing process. Typically the Q would have to be about 50 to ensure sufficient attenuation of the image. Such a narrowband filter at high frequencies such as 1100 MHz are expensive and often necessitate the use of a matching network to properly interface to both the output of the first mixer 24 and to the input of the second mixer 28.
Since the dual conversion architecture 20 employs two mixing steps, there could be additional distortion and phase noise compared to the single conversion tuner architecture 10. Each mixing step could introduce distortion due to the mixing process. The phase noise present in the local oscillator signal(s) could also degrade the signal integrity.